Method of Producing a Porous Semiconductor Film on a Substrate

ABSTRACT

The invention relates to a method of producing a porous semiconductor film and the film resulting from such production. It furthermore relates to an electronic device incorporating such film and to potential uses of such film.

The invention relates to a method of producing a porous semiconductorfilm and the film resulting from such production. It furthermore relatesto an electronic device incorporating such film and to potential uses ofsuch film.

Single crystal solar cells show energy conversion efficiencies as highas ˜25%. Where the Si-based crystals are no longer single crystals butpolycrystalline, the highest efficiencies are in the range of ˜18%, andwith amorphous Si, the efficiencies are ˜12%. Solar cells based on Siare, however, rather expensive to manufacture, even in the amorphous Siversion. Therefore alternatives have been developed based on organiccompounds and/or a mixture of organic and inorganic compounds, thelatter type solar cells often being referred to as hybrid solar cells.Organic and hybrid solar cells have proved to be cheaper to manufacture,but seem to have yet comparably low efficiencies even when compared toamorphous Si cells. Due to their potential inherent advantages such aslight weight, low-cost fabrication of large areas, environmentallyfriendly materials, or preparation on flexible substrates, efficientorganic devices might prove to be technically and commercially useful“plastic solar cells”. Recent progress in solar cells based ondye-sensitised nanocrystalline titanium dioxide (porous TiO₂)semiconductor and a liquid redox electrolyte demonstrates thepossibility of a high energy conversion efficiencies in organicmaterials. (B. O-Regan and M. Grätzel, Nature 353 (1991, 737).

Photoelectrochemical cells based on sensitisation of nanocrystallineTiO₂ by molecular dyes (dye sensitised solar cells, DSSC) have attractedgreat attention since their first announcement as efficient photovoltaicdevices (B. O'Regan and M. Grätzel, see above; WO 91/16719). One part ofthe ongoing investigations is to exploit the potential applicability ofsuch cells on flexible substrates and with this the potential offabricating flexible solar cells. One of the main challenges to besolved prior to the successful introduction of such flexible DSSC is therestricted range of temperature applicable to plastic substrates.Mostly, the used TiO₂ nanoparticles are brought into good electricalcontact by the application of temperatures as high as 450° C. Suchprocesses are not applicable on flexible plastic substrates which limitsso far the efficiency of cells made of these substrates. With respect toother sintering methods, the most promising way to fabricate flexibleDSSCs has been so far to apply high pressures to the TiO₂ layer [H.Lindström, et al., A new method for manufacturing nanostructuredelectrodes on plastic substrates, Nano Lett. 1, 97 (2001); H. Lindström,et al., Method for manufacturing nanostructured thin film electrodes. WO00/72373; H. Lindström, et al., A new method to make dye-sensitizednanocrystalline solar cells at room temperature, J. Photochem.Photobiol. A 145, 107 (2001); G. Boschloo, et al., Optimization ofdye-sensitized solar cells prepared by compression method, J. Photochem.Photobiol. A 148, 11 (2002).]. Additionally, chemical sintering has beenapplied with minor success [D. Zhang, et al., Low-temperaturefabrication of efficient porous titania photoelectrodes by hydrothermalcrystallization at the solid/gas interface, Adv. Mater. 15, 814 (2003);D. Zhang, et al., Low temperature synthesis of porous nanocrystallineTiO ₂ thick film for dye-sensitized solar cells by hydrothermalcrystallization, Chem. Lett. 9, 874 (2002).]. Combination of bothmethods, i.e. temperature sintering and chemical sintering leads alsoonly to minor improvements [S. A. Hague, et al., Flexible dye sensitisednanocrystalline semiconductor solar cells, Chem. Comm. 24, 3008 (2003)].

The disadvantages of the state of the art of fabricating flexible solarcells can be summarized as follows:

High temperature sintering is used in order to provide a good electricalcontact between the semiconductor particles. However, the temperaturesrequired for good electrical contact between the nanoparticles are farhigher than can be tolerated by most flexible, i.e. for examplepolymeric, substrates on which the components of a “plastic solar cell”would have to be applied. Therefore due to the inherent material limits,process parameters need to be compromised with, which effectively fromthe beginning limits the performance of the solar cell thus produced.

If on the other hand, low temperatures are used (around 200° C.) forsintering, or sintering α-curs additionally or alternatively by theapplication of high pressures, one cannot use organic binders in theinitial material. Usually, these organic binders are used in order tocontrol the porosity of the layer containing the semiconductorparticles. Subsequently, in the high temperature methods, the organicbinders are simply burned away leaving a void space behind. In the lowtemperature sintering processes, however, these binders cannot be used,since they would simply not be burned away. Therefore, the porosity and,concomitantly therewith, the ionic transport through the pores iseffectively decreased. Additionally, the electrical contact between theparticles does not reach the quality compared to hot sintered layers. Acombination of low temperature sintering and the application of pressureimproves the electrical contact to some extent, but the problem of lowporosity remains unsolved.

In chemical sintering processes, low temperature activated ornon-activated chemical reactions resulting in oxide layers, are used toovercoat the nanoparticles in the porous layers. They form a conductiveouter layer, which improves the electrical conductivity of the porousfilm. However, those films are expected to have a high defectconcentration. In addition, it is not clear if transport only occurs inthe thin overlayer. In both cases, this leads to a lower performance athigher light intensities, as reported in [Hague et al., see above]. Thisstrongly limits the applicability of those cells.

Accordingly it was an object of the present invention to provide for amethod of production that allows to make use of the advantages of thehigh temperature sintering processes in combination with a flexiblesubstrate, which, as such would not tolerate high temperature sinteringprocesses. Furthermore it was an object of the present invention toprovide for a plastic solar cell which can be produced in an inexpensivemanner. Furthermore it was an object of the present invention to providefor a plastic solar cell which has efficiencies at least comparable tothose reported about in the literature.

All these objects are solved by a method of producing a poroussemiconductor film on a substrate, comprising the steps:

-   a) preparing, on a first substrate, an adhesion layer, capable of    providing electrical and mechanical contact between a porous    semiconductor layer attached to said adhesion layer, and said first    substrate,-   b) preparing a porous semiconductor layer on a second substrate,-   c)transferring said porous semiconductor layer onto said adhesion    layer, and optionally, after step b) or c),    preparing a second, third, fourth, fifth . . . n-th porous    semiconductor layer on a third, fourth, fifth . . . n-th, (n+1)-th    substrate, and transferring said second, third, fourth, fifth, . . .    n-th porous semiconductor layer onto said first, second, third,    fourth, . . . and (n−1)-th porous semiconductor layer respectively,    n being an integer from 2 to 100, preferably from 2 to 20, more    preferably from 2 to 10, and furthermore, optionally, preparing on    one, some or each of the second, third, fourth, fifth . . . n-th    porous semiconductor layer, further adhesion layer(s) onto which the    respective subsequent semiconductor layer is transferred.

As an example, if a second porous semiconductor layer is prepared on athird substrate it will be transferred onto said first poroussemiconductor layer which itself has been transferred onto said adhesionlayer. Additionally, there may also be present an additional adhesionlayer between said first and said second porous semiconductor layer. Ina preferred embodiment, the porous semiconductor film on a substratecomprises a sequence of alternating adhesion layers and poroussemiconductor layers which are stacked on top of each other and whichhave been prepared by the aforementioned steps a) to c) and possibly thesubsteps ba), bb), bc), ca), cb) and cc)(see below for these substeps).

In one embodiment said adhesion layer is transparent, semi-transparentor opaque. Preferably it is transparent. In another embodiment it isopaque and thus more light-scattering.

In one embodiment, said porous semiconductor layer is transparent,semi-transparent or opaque.

In one embodiment said second, third, fourth etc. semiconductor layer istransparent, semi-transparent or opaque. In one embodiment subsequentsemiconductor layers are increasingly opaque, thus causing a greaterscattering. In one embodiment, the opacity of an individual poroussemiconductor layer changes, preferably increases, over the respectiveindividual layer's thickness. This may apply in films comprising onlyone porous semiconductor layer, or in films comprising several poroussemiconductor layers.

In one embodiment step c) comprises the steps:

-   ca) separating said porous semiconductor layer from said second    substrate,-   cb) optionally dyeing said porous semiconductor layer, preferably    using a dye useful for dye sensitized solar cells,-   cc) transferring said porous semiconductor layer without said second    substrate onto said adhesion layer.

Preferably, step b) comprises the steps:

-   ba) preparing said porous semiconductor layer on said second    substrate by a method selected from printing, in particular screen    printing, doctor blading, drop casting, spin coating, ink-jet    printing and spraying,-   bb) sintering said porous semiconductor layer, and optionally,-   bc) dyeing said porous semiconductor layer, preferably using a dye    useful for dye sensitised solar cells.

In one embodiment in step a) said adhesion layer is prepared on saidfirst substrate by a method selected from printing, in particular screenprinting and/or ink-jet-printing, doctor blading, drop casting, spincoating, sputtering, sol gel methods, and spraying.

In a further embodiment said adhesion layer has also the function ofbeing a blocking layer between said first substrate and thelater-applied electrolyte to prohibit direct contact between the two. Tofulfill such a function, said adhesion layer might be composed of twosublayers, one of the sublayers, preferably the lower sublayer being theblocking layer and the other sublayer being the adhesion layer. Thelower part (blocking layer) may be prepared, among others, by means ofsputtering methods, preferably suitable for plastic substrates, or solgel methods.

Preferably, step ca) comprises the lifting-off of said poroussemiconductor layer from said second substrate, wherein, preferably, thelifting-off occurs by removal of said second substrate or parts of itfrom said porous semiconductor layer, and wherein, more preferably, saidremoval is performed by physical methods, e.g. peeling, and/or chemicalmethods, e.g. etching and/or oxidation.

In one embodiment said transfer of step c) is performed, while saidporous semiconductor layer is in a wet or dry state, wherein,preferably, said transfer is achieved by a roll-to-roll

In one embodiment, the method according to the present invention,additionally comprises the step

-   d) sintering and/or pressing of a composite, comprising, in that    order and on top of each other, said first substrate, said adhesion    layer, and said porous semiconductor layer.

In one embodiment said sintering of step bb) occurs at a temperature inthe range of from 300° C.-500° C., preferably >350° C., morepreferably >380° C., most preferably >400° C.

In one embodiment said sintering in step d) occurs at a temperature inthe range of from 50° C. to ≦200° C., and/or said pressing occurs with apressure in the range of from 0-12×10⁴ N/cm².

Preferably, said adhesion layer is a layer of semiconductor particles,preferably oxide particles, more preferably TiO₂-particles, inparticular anatase-TiO₂ particles.

It is clear to someone skilled in the art that a wide variety ofsemiconductor particles can be used for practicing the presentinvention. Examples of these are, without being limited thereto: TiO₂,SnO₂, ZnO, Nb₂O₅, ZrO₂, CeO₂, WO₃, SiO₂, Al₂O₃, CuAlO₂, SrTiO₃ andSrCu₂O₂, or a complex oxide containing several of these oxides.

In one embodiment said porous semiconductor layer is a layer ofsemiconductor particles, preferably oxide particles, more preferablyTiO₂-particles, in particular anatase-TiO₂ particles, wherein,preferably, said porous semiconductor layer comprises semiconductorparticles having sizes in the range of from about 10 nm to 1000 nm,preferably 10 nm to 500 nm.

In one embodiment said porous semiconductor layer has a porosity in therange of from 30% to 80%, as measured by nitrogen adsorption techniques.In this context and as used herein, a film having x % porosity meansthat x % of the total volume occupied by the film are void space.

In one embodiment said porous semiconductor layer is a composite layercomprising a first sublayer and a second sublayer adjacent to said firstsublayer, wherein said first sublayer comprises spherical nanoparticlesand said second sublayer comprises elongated rod-like nanoparticles.Preferably, said nanoparticles are semiconductor nanoparticles.Preferably, said spherical nanoparticles have a size in the range offrom about 10 nm to about 500 nm, more preferably about 10 nm to about250 nm, and said elongated rod-like particles have an average lengthalong their longest dimension of 10 nm to 500 nm, preferably around 50nm to 200 nm. In one embodiment, the ratio between the longest axis andshortest axis of the elongated rod-like particles is 2 or larger. In apreferred embodiment, the ratio between the longest axis and shortestaxis is between 2 and 10.

In another embodiment, said rod-like nanoparticles are fibers with aratio between longest axis and shortest axis between 20 and 1000.

In one embodiment, said first sublayer with spherical nanoparticlescontains also rod-shaped nanoparticles and said second sublayer withrod-shaped nanoparticles contains also spherical nanoparticles, howeverthe content of rod-shaped particles in the first layer is less than thecontent of rod-shaped particles in the second layer and the content ofspherical nanoparticles in the second layer is less than the content ofspherical particles in the first layer. Whenever sublayers are referredto in this application as “sublayers containing spherical nanoparticles”or “sublayers containing elongated rod-like particles” these terms alsoinclude the aforementioned possibility of the respective sublayercontaining also nanoparticles of the “other” kind.

In one embodiment, however, said first sublayer of sphericalnanoparticles exclusively contains spherical nanoparticles and saidsecond sublayer of elongated rod-like nanoparticles exclusively containselongated rod-like nanoparticles.

In one embodiment, said porous semiconductor layer is composed of amixture of spherical and elongated rod-like nanoparticles with thecontent of the rod-like nanoparticles gradually increasing from thebottom of the layer to the top of the layer, or vice versa.

In one embodiment the rod-like nanoparticles are nanotubes.

In one embodiment, in said porous semiconductor film on a substrate,said first sublayer is facing said adhesion layer and said secondsublayer is further removed from said adhesion layer In anotherembodiment, in said porous semiconductor film on a substrate, saidsecond sublayer is facing said adhesion layer and said first sublayer isfurther removed from said adhesion layer.

Preferably, said first and said second sublayer have a thickness in therange of from 1 μm to 20 μm each.

Preferably, said second substrate is a substrate capable of withstandingtemperatures >350° C., preferably >400° C., and wherein, preferably,said second substrate is made of glass or metal, preferably steel, orglass with one or several additional layers on top of it, e.g. metallayers.

In one embodiment said second substrate additionally comprises a spacerlayer, upon which said porous semiconductor layer is prepared, wherein,preferably said spacer layer can be removed by chemical and/or physicalmethods, thereby allowing the lifting-off of said porous semiconductorlayer.

In one embodiment said spacer layer is organic, inorganic, metal,preferably gold, or a combination thereof, wherein, preferably, saidspacer layer is made of gold, and removal thereof occurs by oxidation,e.g. by treatment with an oxidising agent, for example a strong acid, ora redox couple, e.g. iodine/iodide.

Preferably, said porous semiconductor layer has a thickness in the rangeof from about 1 μm to about 50 μm.

In one embodiment said adhesion layer is a layer of semiconductorparticles having sizes in the range of from about 10 nm to about 100 nm,preferably 10 nm-50 nm, more preferably 10 nm-20 nm.

Preferably, said adhesion layer has a thickness in the range of from 10nm to 1 μm, preferably a thickness <500 nm, more preferably ≦100 nm.

In one embodiment said first substrate is made of flexible material,which, preferably is incapable of withstanding sintering procedures attemperatures >250° C.

The objects of the present invention are also solved by a poroussemiconductor film, produced by the method according to the presentinvention.

The objects are furthermore solved by a porous semiconductor film,preferably produced by a method according to the present invention,comprising, on a substrate:

-   -   a sequence of alternating adhesion layers and porous        semiconductor layers on top of each other, said layers being as        defined above, preferably with said porous semiconductor layers        having been dyed or dye sensitized with one or several dyes,        with subsequent porous semiconductor layers, due to the presence        of a respective dye, having the center of mass of absorption        shifted to longer wavelengths than the previous porous        semiconductor layer, said previous porous semiconductor layer        being closer to the substrate.

The terms “dyed” and “dye sensitized”, as used herein, are usedinterchangeably.

In one embodiment, the porous layers are dyed before the transfer ontothe corresponding adhesion layers. The absorption range of the dyedporous layers may vary. Preferably, the first porous layer absorbs in ashorter wavelength region with the center of mass of absorption of thesubsequent layers shifting subsequently to longer wavelengths.

They are furthermore solved by a porous semiconductor film, preferablyproduced by the method according to the present invention, comprising,in that order:

-   -   a first substrate, preferably a flexible substrate, which, more        preferably, is incapable of withstanding sintering        temperatures >250° C.,    -   an adhesion layer, capable of providing electrical and        mechanical contact between a porous semiconductor layer attached        to said adhesion layer, and said first substrate, said adhesion        layer being a layer of semiconductor particles, preferably in        the range of from 10 nm to 100 nm, more preferably 10 nm to 50        nm, most preferably, 10 nm to 20 nm, having a porosity of        30%-80%, with an average pore size in the range of from 1 nm to        about 100 nm.    -   a porous semiconductor layer, comprising semiconductor particles        having sizes of from about 10 nm to about 1000 nm, and having a        pore size in the range of from about 3 nm to about 500 nm, said        porous semiconductor layer having a thickness in the range of        from about 1 μm to about 50 μm and a porosity in the range of        from 30% to about 80%, as measured by nitrogen adsorption        techniques.

The objects of the present invention are also solved by an electronicdevice comprising a porous semiconductor film according to the presentinvention, wherein, preferably, said electronic device is a solar cell,and wherein, more preferably, said solar cell has a power conversionefficiency of >5%. In one embodiment said solar cell has a relativeporosity of ≧75%, preferably ≧80%, after a pressure of up to 6×10⁴N/cm², preferably up to 10×10⁴ N/cm² has been applied to said poroussemiconductor film, said relative porosity being defined with respect tothe unpressed film. In another embodiment, said electronic device is asensor device.

The objects of the present invention are furthermore solved by the useof the method according to the present invention for producing anelectronic device, in particular a solar cell.

They are also solved by the use of the porous semiconductor filmaccording to the present invention in an electronic device, preferably asolar cell.

The objects of the present invention are also solved by a poroussemiconductor layer comprising a first sublayer of sphericalnanoparticles and a second sublayer of elongated rod-like nanoparticlesadjacent to said first sublayer. Preferably, said nanoparticles aresemiconductor nanoparticles. Such a semiconductor layer comprising afirst sublayer of spherical nanoparticles and a second sublayer ofelongated rod-like nanoparticles adjacent to said first sublayer is alsosometimes herein referred to as a “composite layer”.

Preferably such a composite layer of spherical and rod-likenanoparticles exhibits a difference in the thermal expansion coefficientbetween the two different sublayers.

Such a composite layer is herein also sometimes referred to as a“sphere-rod composite layer” or “SRCL” As used herein, the term“sphere-rod composite layer” is meant to designate any layer comprisinga sublayer of spherical nanoparticles and another sublayer of elongatedrod-like particles, adjacent to it. It is clear to someone skilled inthe art that the terms “spherical” and “elongated rod-like” are onlyapproximative terms and can be used to also describe particles which arenot entirely “spherical” or not entirely “rod-like” in a strictgeometrical sense, yet whose appearance is still adequately described bythese terms.

The term “adjacent to” is meant to designate the spatial arrangement oftwo entities which are next to each other. In one embodiment, the twosublayers adjacent to each other are in direct contact with each other.In another embodiment the two sublayers adjacent to each other areseparated by an intermediate spacer layer, the dimensions of which maybe very small in comparison to the thickness of each sublayer. In apreferred embodiment, however, the two sublayers adjacent to each otherare in direct contact with each other.

In one embodiment, said spherical nanoparticles have a size in the rangeof from about 10 nm to about 500 nm, more preferably 10 nm to about 250nm, and said elongated rod-like particles have an average length alongtheir longest dimension of 10 nm to 500 nm, preferably around 50 nm to200 nm. In a preferred embodiment, the ratio between the longest axisand the shortest axis of the elongated rod-like particles is 2 orlarger. In a more preferred embodiment, the ratio between the longestaxis and the shortest axis is between 2 and 10.

In one embodiment, said composite layer of spherical and rod-likeparticles has a thickness in the range of from about 2 μm to 50 μm,preferably from about 2 μm to about 40 μm.

Preferably said semiconductor particles are oxide particles. Examples ofthese are, without being limited thereto: TiO₂, SnO₂, ZnO, Nb₂O₅, ZrO₂,CeO₂, WO₃, SiO₂, Al₂O₃, CuAlO₂, SrTiO₃ and SrCu₂O₂

More preferably said semiconductor particles are TiO₂-particles, inparticular anatase-TiO₂-particles.

In one embodiment, said sphere-rod composite layer has a porosity in therange of from 30% to 80%, as measured by nitrogen adsorption techniques.In this context and as used herein, a film or a layer having x %porosity means that x % of the total volume occupied by the film orlayer are void space.

The inventors have surprisingly found that such a SRCL can be lifted offa substrate much more easily in comparison to non-composite layers.Hence, such a SRCL can be used and applied in any method involving thelifting of a layer, preferably a semiconductor layer off a substrate.

The present inventors have also surprisingly found that theabove-mentioned disadvantages in fabricating flexible DSSCs can beovercome in general by the application of a transfer method for theporous TiO₂ layer. This method combines the advantage of the hottemperature sintering of the active porous layer (good electricalcontact, good porosity, good mechanical stability) with the possibilityto apply such a porous layer on flexible plastic substrates and to bringthem in good electrical contact. It ensures the full freedom in thechoice of the parameters of the porous layer to be transferred and withthis optimal electrical and optical properties of the layer. Itsprinciple is based on the separation of layer preparation and contactingto the substrate. That means that the active porous layer is prepared ona substrate, which can withstand high temperatures (spare substrate).After successful preparation of this layer, it is removed from the sparesubstrate and transferred onto another substrate (e.g. incapable ofwithstanding high temperature sintering processes). To bring it in goodmechanical and electrical contact with this other substrate (the “first”substrate according to the terminology of the appended claims), a spaceror adhesion layer exists between the flexible substrate and thetransferred porous layer which spacer or adhesion layer is capable ofproviding electrical and mechanical contact between the flexiblesubstrate and the transferred porous layer. As used herein, the term“adhesion layer” is meant to denote any layer which provides adhesionbetween a substrate (which itself may for example be covered by a TCOlayer) and another layer, preferably a porous semiconductor layer, to beaffixed to said substrate, or it provides adhesion between two adjacentlayers, preferably two subsequent porous semiconductor layers. It hasbeen found that good results can be obtained when this spacer layer oradhesion layer consists of a very thin layer of nanoporous semiconductorparticles, preferably TiO₂ particles. To bring the transferred porouslayer (herein also sometimes referred to as “transfer layer”), thenanoporous adhesion layer and the substrate into good electricalcontact, different low temperature sintering processes, as e.g. heatingto only 200° C., pressing, and/or chemical sintering are sufficient andmay be applied. Since the adhesion layer is much thinner than a standardporous layer employed in DSSC, the above-mentioned disadvantages ofthose methods are much less important than if applied to the activeporous layer. E.g., pressing does not affect the good properties of thetransfer layer but enhances the electrical contact between thenanoparticles of the adhesion layer and the contact between theelectrode, the adhesion layer, and the transfer layer. Best results havebeen found so far for a combination of heating and pressing.

It is clear to someone skilled in the art that the semiconductorparticles used in the porous semiconductor layer (herein also sometimesreferred to as “transfer layer”) and/or the adhesion layer may take anyshape, unless explicitly specified as in the case of the SRCLs. They maybe for example spheres, rods or tubes. They may take any aspect ratio,and different diameters and shapes may be mixed. Likewise mixedmultilayer-structures may be applied. In one embodiment, subsequentlayers, i.e. the adhesion layer and subsequent semiconductor layers maybe increasingly scattering; i.e. less transparent than the immediatelypreceding neighbouring layer. This increasing scattering effect may bedue to the different composition of each layer, with subsequent layershaving differently sized and/or shaped particles, and the mean sizes ofthe particles increase from layer to layer. Preferably, the surfaceroughness of the respective layer should not be substantially largerthan the size(s) of the smaller particles, preferably the size(s) of thesmaller particles of the respective layer. This is to ensure a smoothsurface transition and a good contact between the two layers.Furthermore, it is possible, to add fibres into the porous semiconductorlayer for a better stability of this layer. The preferred thickness ofthe porous semiconductor layer is about 1 μm to about 50 μm.

The porous semiconductor layer, at first, may be prepared on any kind ofsubstrate, which is preferably, flat and temperature resistant, i.e. itcan withstand high temperature sintering processes, such as are used forsintering semiconductor layers (e.g. temperature range from 350° C.-500°C.). Alternatively, this substrate may simply resemble the shape or havethe same shape as the substrate, on which the adhesion layer isprepared. A preferred substrate material is glass or steel, but otherscan be used. The porous semiconductor layer may be prepared by anysuitable means including but not limited to printing, in particularink-jet printing, screen printing, doctor blading, drop casting, spincoating and spraying. One preferred method of preparing the poroussemiconductor layer is ink-jet printing, since in this method controlcan be precisely exerted on the thickness of the layers applied, andextremely thin layers corresponding to about 1-5 monolayer, if desired,can be generated, with one monolayer corresponding to the thickness of asingle particle. However, ink-jet printing is also perfectly suitablefor preparing thicker layers in the μm-range, such as for example from 2μm to 50 μm. Another particularly preferred method for preparing theporous semiconductor layer is screen printing. One method oftransferring the porous semiconductor layer from the substrate on whichit has been produced, to the adhesion layer is the presence of a spacerlayer on the substrate on which the semiconductor layer has beenproduced. This spacer layer may facilitate the removal of the poroussemiconductor layer from the substrate on which it was produced, afterthe porous semiconductor layer has been sintered. Such a spacer layermay comprise organic or inorganic materials, it may comprise metals, inparticular gold. The spacer layer may be removed by chemical, physicalor other methods, which are apparent to someone skilled in the art. Forexample gold may be oxidised by treatment with an oxidising agent, forexample a strong acid, and/or a redox-couple, e.g. iodine/iodide. As aresult of such removal, the porous semiconductor layer may be lifted offthe substrate on which it had been produced and be transferred to theadhesion layer. The porous semiconductor layer is the layer which is themain “active” layer of the electronic device, meaning the location wherethe light absorption, charge separation and transfer processes mainlytake place.

In preferred embodiment, the semiconductor layer is a composite layercomprised of two different sublayers of spherical and rod-likesemiconductor nanoparticles, respectively. Such a SRCL (sphere-rodcomposite layer) can be lifted off a substrate much more easily andtherefore facilitates any process wherein a lifting off a semiconductorfilm from a substrate is re

The present inventors found that the lift-off process described above issignificantly facilitated by the application of sphere-rod compositelayers (SRCL) as shown in FIG. 6. Without wishing to be bound by anytheory, the effect can be described as follows:

Firstly, the inventors measured the time necessary for the removal ofthe porous layers from the spare substrate in iodine/iodide electrolytewithout any additional mechanical force other than gravity for thefollowing: SRCLs, homogeneous porous layers, and porous layersconsisting of two sub-layers—the latter consisting of either smallparticles of identical geometrical shape (10 to 20 nm in diameter) inboth sub-layers or small particles in the lower sublayer and biggerparticles of the same geometrical shape (300 nm in diameter) in theupper sub-layer. The average time for all the reference layers exceededthe average time for the SRCLs by at least one order of magnitude. Thecomparison of data for SRCLs and double layers consisting of twosub-layers of nanospheres is shown in FIG. 8. The inventors proposethat, without wishing to be bound by any theory, the most likelyexplanation for these observations is that internal stresses in theSRCL, due to different expansion/contraction during the formation of thelayer in the high temperature sintering process and during thesubsequent cooling down, may cause the easier removal from the sparesubstrate. However, since both spheres and rods in the particularembodiment used for exemplification, are anatase TiO₂ single crystals ashas been confirmed by X-ray diffraction, the effect cannot be purely dueto different crystallographic properties in the two layers but must beinstead correlated to the nano-morphology of the film. Indeed, whereasclosed-packed structures consisting of either randomly arranged bulkpolycrystalline material, single crystalline rods, or single crystallinespheres, are expected to exhibit the same averaged thermal expansioncoefficient, yet nanoporous structures allow for a deviation dependingon the shape of the nanoscale building blocks as will be explained inthe following. The TiO₂ nanorods are grown by means of a templatemethod. It is based on the preferential adsorption of some surfactantson surfaces of selected crystallographic orientation. In the case ofdiethylenetriamine, as used here for the nanorod synthesis, adsorptiontakes place preferentially on planes parallel to the [001] direction orc-axis of the anatase lattice (see also FIG. 7 for the nomenclature ofaxes and directions). Since the surfactants slow down the crystal growthalong the direction perpendicular to those planes, the longer axis ofall nanorods is expected to coincide (within some deviation) with the[001] direction, and this indeed has been verified by means oftransmission electron microscopy. On the other hand, the thermalexpansion coefficients, α, for anatase TiO₂ are known to be stronglydependent on crystallographic orientation with even negative expansionalong the a-direction, i.e. perpendicular to [001] (α_(a)=−2.88×10⁻⁶,α_(c)=6.6424×10⁻⁶, both at room temperature). In the one-dimensionalmodel of FIG. 7 (one-dimensional in the sense that the model onlyaccounts for different expansion coefficients along the chain ofnanoparticles) with the c-axis of either the spherical or the rod-likeparticles oriented randomly in all three directions, there is adifference in the macroscopic thermal expansion coefficient because thelong axis of the rods contributes more to the total length of the chainof nanoparticles as shown in FIG. 7) than the short axis. Although thismodel oversimplifies the situation in the nanoporous network, based onthese considerations a difference in the macroscopic thermal expansioncoefficient is expected for the porous layers consisting of eitherspheres or rods. Furthermore, because the layer structure is formedduring the sintering cycle at 450° C., even small differences in α aresufficient to account for the internal stress observed at roomtemperature. Indeed, when the inventors measured the thickness of porouslayers between room temperature and 300° C. by means of opticalinterferometry, they found a different behaviour of standard layers andSRCLs. For the standard layers, a decreasing thickness with increasingtemperature is observed. This is attributed to a shrinking of the layerdue to a stronger lateral expansion of the glass substrate when comparedwith the film itself. However, for the nanorod layers, they do notobserve a shrinking which can be interpreted in terms of a more positiveexpansion coefficient of the nanorod layers. In short, the shape of theTiO₂ on the nanometer scale, in combination with the nanoporousarrangement of the particles in the layer, are correlated to themacroscopic differences in the mechanical properties of the layersleading to the optimization of the lift-off process.

As far as the adhesion layer is concerned, which itself does not have totake part in the active processes of the electronic device in which theporous semiconductor film is to be used, this adhesion layer may becomprised of any material which may be formed into a thin, transparentlayer and which can supply a good electrical and mechanical contactbetween the porous semiconductor layer and the substrate (for example anelectrode). The material of the adhesion layer may be nanoporous,meaning that the average pore size within the adhesion layer is in therange of from 1 nm to 100 nm. In a preferred embodiment, the nanoporousparticles of the adhesion layer are semiconductor particles, preferablyoxide particles, more preferably TiO₂-particles. It is clear to someoneskilled in the art, that also these particles may take any shape, forexample they may be spheres, rods or tubes. Their preferred sizes are inthe range of from about 10 nm to about 100 nm, but it has to beemphasised, that, preferably, the surface roughness of the respectivelayer should not be substantially larger than the size(s) of the smallerparticles, preferably the size(s) of the smaller particles of therespective layer. Again at the interface between the adhesion layer andthe porous semiconductor layer, the particles of the adhesion layer arerelatively small in order to ensure a smooth surface and a good contactbetween the two layers. The preferred thickness of the adhesion layer isbetween about 10 nm to about 1 μm, but embodiments with a thickness <500nm are preferred. Embodiments having a thickness ≦100 nm are even morepreferred. The adhesion layer may be applied by any means, including butnot limited to printing, in particular ink-jet printing, screenprinting, doctor blading, drop casting, spin coating and spraying. Onepreferred method of applying the adhesion layer is ink-jet-printing,since in this method, control can be precisely exerted on the thicknessof the layers applied, and extremely thin layers, corresponding to about1-5 monolayers can be generated, with one monolayer corresponding to thethickness of a single particle. In principle, any substrate for theadhesion layer is possible, but for the purpose of producing “plasticsolar cells”, of course, relatively flexible substrates are preferred.It is clear to someone skilled in the art, that a number of materialscan be used for such flexible substrates, including but not limited topolymeric materials. Also, the substrate may be flat or take on anyother shape. In a preferred embodiment, when the substrate is a flexiblepolymeric substrate, it only can withstand temperatures up to a certainlimit, for example 200° C.

The transfer of the porous semiconductor layer from the first substrateon which it has been produced to the substrate which is to be the onealso used in the electronic device, any technique suitable for thatpurpose can be applied. Such techniques are known to someone skilled inthe art and include, but are not limited to roll-to-roll-techniques.During transfer, the porous semiconductor layer may be in a wet or drystate. After the porous semiconductor layer has been transferred to theadhesion layer a low temperature sintering and/or the application ofpressure may ensue. Preferred temperature values for low temperaturesintering are ≦200° C. Preferred pressures are in the range of 2×10⁴N/cm²-12×10⁴ N/cm². It is clear to someone skilled in the art that thepresent invention is not limited to a single adhesion layer and a singleporous semiconductor layer. The present inventors also envisage othercomposites with more than one semiconductor layer and/or more than oneadhesion layer.

Once a composite comprising at least a substrate, a transparent adhesionlayer and a porous semiconductor layer has been produced, this may beused for the production of for example a solar cell. Solar cells andtheir other components are known to someone skilled in the art, i.e. itwill include an electrolyte, the porous semiconductor layer will betreated with a dye in order to dye-sensitise it. As electrode materialthose materials may be used which are not temperature resistant, becausethey do not need to be subjected to high temperature sinteringprocesses. Such materials for making up the electrodes are known tosomeone skilled in the art and comprise but are not limited to metals,organic materials, e.g. highly doped poly(3,4-ethylene dioxide thiophen)(PEDOT or PEDT), and derivatives and TCO-layers (transparent conductiveoxide). The same applies for the counter electrode which may be anorganic material, a TCO-material, and/or a metal, for example platinum.Exemplary TCO-materials are, without being limited thereto, FTO, ITO,ZnO, SnO₂ and combinations thereof.

It is also clear to someone skilled in the art that there exist a widevariety of flexible substrates. For example, flexible, mainly polymeric(with the exception of steel) substrates may be used, such as but notlimited thereto: polyethylene terephthalate (PET), polyethylenenaphthalate (PEN), polyethersulfone (PES), polyimide (Kapton),polyetheretherketone (PEEK), polyetherimide (PEI), stainless steel, OHP(overhead transparencies).

Reference is now made to the figures wherein

FIG. 1 shows the schematics of a cell made by means of the transfertechniques/lift-off-techniques described according to the presentinvention;

FIG. 2 shows the I-V-characteristics of a lift-off-fabricated DSSC, i.e.produced according to the present invention. The temperature applied forsintering of the adhesion layer was 200° C., the applied pressure was 60kN/cm²; measured with simulated sunlight at 100 mW/cm², AM 1.5;

FIG. 3 shows the efficiency and short circuit current density J_(sc) ofDSSC according to the present invention as a function of appliedpressure. The pressing temperature was room temperature; measured usinga sulfur lamp with white light, at 100 mW/cm²;

FIG. 4 shows the efficiency and relative porosity of DSSC according tothe present invention as a function of applied pressure. The pressingtemperature was room temperature. 100% relative porosity means theporosity of a cell before any external pressure is applied;

FIG. 5 shows the efficiency of lift-off-cells according to the presentinvention as a function of adhesion layer thickness. Cells were sinteredat 200° C. only, no pressure was applied.

FIG. 6 shows a schematic diagram and a scanning electron micrograph of aSRCL according to the present invention;

FIG. 7 shows the schematics of a chain of TiO₂-nanospheres and a chainof TiO₂ nanorods. The [001] direction contributes more in the case ofthe nanorods;

FIG. 8 a) shows the schematics of an improved lift-off, presumably dueto a different thermal expansion in the nanorod sublayer in comparisonto the nanosphere sublayer.

FIG. 8 b) shows a comparison of the time needed for lifting off standarddouble layers (having particles of different sizes but like shape) andsphere-rod composite layers according to the present invention.

The invention will now be further described by reference to thefollowing examples which are presented to illustrate, not to limit thepresent invention.

EXAMPLE 1

A schematic view of the DSSC made by means of the above describedlift-off techniques is shown in FIG. 1. In a typical cell producedaccording to the method of the invention, the substrate is covered witha transparent conductive oxide layer (TCO, approx. 100 nm). On the TCO,a thin adhesion layer of approx. 100 nm thickness consisting of TiO₂nanoparticles with about 14 nm in diameter ensures the contact betweenthe TCO and the active porous layer (the porous semiconductor layer ortransfer layer) which has been transferred onto the substrate aftersintering on another substrate. The active porous layer consists of adouble structure with an approx. 8 μm thick sub-layer consisting of 20nm particles (Part I in FIG. 1) and a 2 μm thick sub-layer consisting ofa mixture of 20 nm and 300 nm particles (Part II in FIG. 1). Onepossibility to remove the active layer from the spare substrate is tosinter it on a thin gold layer. After sintering, the gold is dissolved,e.g. with an iodine/iodide mixture, and the active layer is transferredin a solvent from the spare to the real substrate. After drying thetransfer and adhesion layer at 85° C. and a low temperature sintering ofthe adhesion layer at 200° C. and 60 kN/cm², red dyemolecules(=(cis-bis(isothiocyanato)bis(2,2′-bipyridyl-4,4′-dicarboxylicacid)ruthenium(II))) are attached as a monolayer to the TiO₂ viaself-assembling out of a solution in ethanol (0.3 mM). The dyesensitised porous layer is filled with a polymer electrolyte (PEO inPC/EC) with iodine/iodide (0.015 M) serving as redox-couple. A 6 μmthick bulk layer of the same polymer electrolyte bridges the gap betweenporous layer and a flat, smooth platinum film (50 nm) applied on asubstrate of any kind. To avoid direct contact between the TiO₂ layerand the platinum counter electrode, inert spacers, e.g. balls made ofglass or spacer foil, are introduced between the two electrodes.

The current-voltage-characteristics of such a solar cell is shown inFIG. 2 for illumination by 100 mW/cm² of simulated sun light (AM 1.5). Amaximum power conversion efficiency of 7% can be extracted from thedata. FIG. 3 shows the efficiency and the short circuit current densityas a function of applied pressure. In contrast to earlier reportedresults on pressed films which were not prepared by means of lift-offtechniques [Lindström et al., 2001, see above], both efficiency andJ_(SC) saturate at pressures not lower than 50 kN/cm². For non-lift-offlayers, best values have been reported for pressures much lower, atabout 5 kN/cm². One reason for this difference might be found in thepressure dependence of the porosity and efficiency of the pre-sintered(450° C.) but non-transferred layers as shown in FIG. 4. Neitherefficiency, nor porosity show a strong dependence on pressure appliedand keep their good properties during pressing. For non-pre-sinteredfilms, however, the porosity decreases strongly with pressure and may beone reason for the worse performance and early saturation of efficiencywith higher pressures of non-sintered but pressed films of largerthickness [Lindström et al., 2001, see above]. The importance of thesmall thickness of the adhesion layer is demonstrated in FIG. 5 wherethe efficiency of lift-off cells is shown as a function of the number ofprinting cycles when being ink-jet printed, each cycle equivalent toabout 100 μm of layer thickness. A strong decrease of efficiency can befound for higher numbers of printing cycles.

EXAMPLE 2

Additionally, experiments on porous transfer layers dye sensitisedbefore lifting-off the transfer layer from the first substrate have beenconducted. The preparation follows a similar route as described for thefirst example but this time, a cell is produced having two adhesionlayers and two transfer layers in alternating fashion on top of eachother, with the transfer layers being dye sensitised. The order oflayers is: substrate-adhesion layer 1-porous semicoductor layer 1(dyed)-adhesion layer 2-porous semiconductor layer 2 (dyed). Thetransfer layers consist of porous TiO₂ layers of about 5 μm thicknessonly and only of particles of about 20 nm in diameter. The transferlayers have been sintered on a thin (approx. 20 nm) gold layer and aftersintering, red dye molecules have been attached as a monolayer to TiO₂via self-assembling out of a solution in ethanol (0.3 mM). The goldlayer was then dissolved by an iodine/iodide mixture. A first porouslayer (porous semiconductor layer 1) was transferred on a substratecovered with TCO (100 nm) and a thin layer (adhesion layer 1) of 100 nmthickness consisting of TiO₂ nanoparticles of about 14 nm in diameter.Adhesion layer 1 was applied by means of ink-jet printing techniques.After drying the first adhesion and transfer layer at room temperature,a second thin adhesion layer (adhesion layer 2) was applied by means ofinkjet printing and a second dye sensitised porous layer (poroussemiconductor layer 2) was transferred onto this second adhesion layer.After a second drying at room temperature, all the layers were sinteredtogether by the application of 60 kN/cm² at room temperature. Powerconversion efficiencies of up to approx. 4% (at irradiation of 100mW/cm²) have been measured for cells fabricated by transfer of suchpre-dyed porous layers. In another experiment, the dye molecules of thesecond porous layer transferred onto the substrate were not red dyemolecules buttri(isothiocyanato)(2,2′:6′,2″-terpyridyl-4,4′,4″-tricarboxylicacid)ruthenium(II) molecules, black dye, with the absorption maximumshifted to longer wave

EXAMPLE 3

Additionally, experiments on porous transfer layers consisting of adouble layer of spheres in the lower and rod-like shaped particles inthe upper part have been performed. These composite layers have beenfabricated as follows: on a thin gold layer on a glass substrate, firstan approximately 5-μm-thick porous layers consisting of 20 nm sphereshas been applied by means of screen printing. After drying at 80° C., anapprox 5-μm-thick porous layer consisting of rods with a length ofapprox 100 nm and a diameter of approx 20 nm (ratio of largest axis:shortest axis=5) has been applied on top of the layer of spheres. Thewhole layer was then sintered at 450° C. Those SRCLs could be lifted offfor further use in DSSC fabricated according to the lift-off techniquesdescribed above, e.g. removal of the gold layer by immersion in agold-dissolving electrolyte. They were also used for the experimentsshown in FIG. 8(b). In these experiments, the lifting-off of sphere-rodcomposite layers was compared to standard double layers consisting ofspheres both in the upper and lower sublayer. The relative film arearemoved from various spare substrates is shown as a function of theduration of immersion of the layers in the gold-dissolving electrolyte;the removal of the sphere-rod composite layers is much easier than ofthe standard layers as indicated by the sharp onset of layer removal at1 min for the SRCL (“composite layers”) but no removal withoutadditional forces for the standard double layers within the time scaleof the experiment.

The features of the present invention disclosed in the specification,the claims and/or in the accompanying drawings, may, both separately,and in any combination thereof, be material for realising the inventionin various forms thereof.

1. A method of producing a porous semiconductor film on a substrate,comprising the steps: a) preparing, on a first substrate, an adhesionlayer, capable of providing electrical and mechanical contact between aporous semiconductor layer attached to said adhesion layer, and saidfirst substrate, b) preparing a porous semiconductor layer on a secondsubstrate, c)transferring said porous semiconductor layer onto saidadhesion layer, and optionally, after step b) or c), preparing a second,third, fourth, fifth . . . n-th porous semiconductor layer on a third,fourth, fifth . . . n-th, (n+1)-th substrate, and transferring saidsecond, third, fourth, fifth, . . . n-th porous semiconductor layer ontosaid first, second, third, fourth, . . . (n−1)-th porous semiconductorlayer respectively, n being an integer from 2 to 100, preferably from 2to 20, more preferably from 2 to 10, and furthermore optionallypreparing on one, some or each of the second, third, fourth, fifth . . .n-th porous semiconductor layer, further adhesion layer(s) onto whichthe respective subsequent semiconductor layer is transferred.
 2. Themethod according to claim 1, wherein step c) comprises the steps: ca)separating said porous semiconductor layer from said second substrate,and cb) optionally dyeing said porous semiconductor layer, preferablyusing a dye useful for dye sensitized solar cells, and cc) transferringsaid porous semiconductor layer without said second substrate onto saidadhesion layer.
 3. The method according to claim 1, wherein step b)comprises the steps: ba) preparing said porous semiconductor layer onsaid second substrate by a method selected from printing, in particularscreen printing, doctor blading, drop casting, spin coating, ink-jetprinting and spraying, bb) sintering said porous semiconductor layer,and, optionally, bc) dyeing said porous semiconductor layer, preferablyusing a dye useful for dye sensitised solar cells.
 4. The methodaccording to claim 1, wherein in step a) said adhesion layer is preparedon said first substrate by a method selected from printing, inparticular screen printing and/or ink-jet-printing, doctor blading, dropcasting, spin coating, and spraying.
 5. The method according to claim 2,wherein step ca) comprises the lifting-off of said porous semiconductorlayer from said second substrate.
 6. A method according to claim 5,wherein the lifting-off occurs by removal of said second substrate orparts of it from said porous semiconductor layer.
 7. The methodaccording to claim 6, wherein said removal is performed by physicalmethods, e.g. peeling, and/or chemical methods, e.g. etching and/oroxidation.
 8. The method according to claim 1, wherein said transfer ofstep c) is performed, while said porous semiconductor layer is in a wetor dry state.
 9. The method according to claim 8, wherein said transferis achieved by a roll-to-roll-technique.
 10. The method according toclaim 1, additionally comprising the step d) sintering and/or pressingof a composite, comprising, in that order and on top of each other, saidfirst substrate, said adhesion layer, and said porous semiconductorlayer.
 11. A method according to claim 3, wherein said sintering of stepbb) occurs at a temperature in the range of from 300° C.-500° C.,preferably >350° C., more preferably >380° C., most preferably >400° C.12. The method according to claim 10, wherein said sintering in step d)occurs at a temperature in the range of from 50° C. to ≦200° C., and/orsaid pressing occurs with a pressure in the range of from 0-12×10⁴N/cm².
 13. The method according to claim 1, wherein said adhesion layeris a layer of semiconductor particles, preferably oxide particles, morepreferably TiO₂-particles, in particular anatase-TiO₂ particles.
 14. Themethod according to claim 1, wherein said porous semiconductor layer isa layer of semiconductor particles, preferably oxide particles, morepreferably TiO₂-particles, in particular anatase-TiO₂ particles.
 15. Themethod according to claim 1, wherein said porous semiconductor layercomprises semiconductor particles having sizes in the range of fromabout 10 nm to 1000 nm, preferably from about 10 nm to about 500 nm. 16.The method according to claim 1, wherein said porous semiconductor layerhas a porosity in the range of from 30% to 80%, as measured by nitrogenadsorption techniques.
 17. The method according to claim 1 wherein saidporous semiconductor layer is a composite layer comprising a firstsublayer and a second sublayer adjacent to said first sublayer, whereinsaid first sublayer comprises spherical nanoparticles and said secondsublayer comprises elongated rod-like nanoparticles.
 18. The methodaccording to claim 17 wherein said spherical nanoparticles have a sizein the range of from about 10 nm to about 500 nm, more preferably about10 nm to about 250 nm, and said elongated rod-like particles have anaverage length along their longest dimension of 10 nm to 500 nm,preferably around 50 nm to 200 nm, wherein, preferably, the ratiobetween the longest axis and shortest axis of the elongated rod-likeparticles is 2 or larger, and, more preferably between 2 and
 10. 19. Themethod according to claim 17, wherein, in said porous semiconductor filmon a substrate, said first sublayer is facing said adhesion layer andsaid second sublayer is further removed from said adhesion layer. 20.The method according to claim 17, wherein said first and said secondsublayer have a thickness in the range of from 1 μm to 20 μm each. 21.The method according to claim 1, wherein said second substrate is asubstrate capable of withstanding temperatures ≧350° C., preferably≧400° C.
 22. The method according to claim 21, wherein said secondsubstrate is made of glass or metal, preferably steel.
 23. The methodaccording to claim 21, wherein said second substrate additionallycomprises a spacer layer, upon which said porous semiconductor layer isprepared.
 24. The method according to claim 23, wherein said spacerlayer can be removed by chemical and/or physical methods, therebyallowing the lifting-off of said porous semiconductor layer.
 25. Themethod according to claim 23, wherein said spacer layer is organic,inorganic, metal, preferably gold, or a combination thereof.
 26. Themethod according to claim 25, wherein said spacer layer is made ofmetal, preferably gold, and removal thereof occurs by oxidation, e.g. bytreatment with an oxidising agent, for example a strong acid, or a redoxcouple, e.g. iodine/iodide.
 27. The method according to claim 1, whereinsaid porous semiconductor layer has a thickness in the range of fromabout 1 μm to about 50 μm.
 28. The method according to claim 1, whereinsaid adhesion layer is a layer of semiconductor particles having sizesin the range of from about 10 nm to about 100 nm, preferably about 10 nmto about 50 nm, more preferably about 10 nm to about 20 nm.
 29. Themethod according to claim 1, wherein said adhesion layer has a thicknessin the range of from 10 nm to 1 μm, preferably a thickness <500 nm, morepreferably ≦100 nm.
 30. The method according to claim 1, wherein saidfirst substrate is made of flexible material, which, preferably isincapable of withstanding sintering procedures at temperatures >250° C.31. A porous semiconductor film, produced by the method accordingclaim
 1. 32. A porous semiconductor film, according to claim 31,comprising, on a substrate: a sequence of alternating adhesion layersand porous semiconductor layers on top of each other, preferably withsaid porous semiconductor layers having been dyed or dye sensitized withone or several dyes, with subsequent porous semiconductor layers, due tothe presence of a respective dye, having the center of mass ofabsorption shifted to longer wavelengths than the previous poroussemiconductor layer, said previous porous semiconductor layer beingcloser to the substrate.
 33. A porous semiconductor film, according toclaim 31, comprising, in that order: a first substrate, preferably aflexible substrate, which, more preferably, is incapable of withstandingsintering temperatures >250° C., an adhesion layer, capable of providingelectrical and mechanical contact between a porous semiconductor layerattached to said adhesion layer, and said first substrate, said adhesionlayer being a layer of semiconductor particles, preferably in the rangeof from 10 nm to 100 nm, more preferably 10 nm to 50 nm, most preferably10 nm to 20 nm, having a porosity of 30% to 80%, with an average poresize in the range of from 1 nm to about 100 nm, a porous semiconductorlayer, comprising semiconductor particles having sizes of from about 3nm to about 1000 nm, and having a pore size in the range of from about10 nm to about 500 nm, said porous semiconductor layer having athickness in the range of from about 1 μm to about 50 μm and a porosityin the range of from 30% to about 80%, as measured by nitrogenadsorption techniques.
 34. A porous semiconductor layer comprising afirst sublayer of spherical nanoparticles and a second sublayer ofelongated rod-like nanoparticles adjacent to said first sublayer,wherein, preferably, said nanoparticles are semiconductor nanoparticles.35. The semiconductor layer according to claim 34, wherein saidspherical nanoparticles have a size in the range of from about 10 nm toabout 500 nm, preferably about 10 nm to about 250 nm, and said elongatedrod-like particles have an average length along their longest dimensionof 10 nm to 500 nm, preferably around 50 nm to 200 nm.
 36. Thesemiconductor layer according to claim 34, wherein the ratio between thelongest axis and the shortest axis of the elongated rod-like particlesis 2 or longer, preferably between 2 and
 10. 37. An electronic devicecomprising a porous semiconductor film according to claim
 31. 38.Electronic device according to claim 37, which is a solar cell or asensor device.
 39. Solar cell according to claim 38, which has a powerconversion efficiency of >5%, and preferably, a relative porosity of≧75%, more preferably ≧80%, after a pressure of up to 6×10⁴ N/cm²,preferably 10×10⁴ N/cm² has been applied to said film, said relativeporosity being defined with respect to the unpressed film.
 40. Use ofthe method according to claim 1 for producing an electronic device, inparticular a solar cell.
 41. Use of the porous semiconductor filmaccording to claim 31 in an electronic device, preferably a solar cell.